The present invention relates to a telephone system designed to be installed at the home of a subscriber who has only one telephone line. The invention more particularly relates to a telephone system comprising several telephone terminals liable to be connected to the single subscriber's line, the functions of this system being to automatically answer a telephone call and to automatically exchange information with the calling party so as to route him to one of the telephone terminals he has selected.
In some telephone interface systems, a device of the telephone answering set type allows picking up the line which amounts to hooking off a telephone handset, further to a telephone call, then to recognize an additional dialling signal emitted on the line by the caller and to consequently switch the telephone line to a desired terminal. To achieve such a function, a telephone interface circuit, arranged between the telephone line and a terminal, is provided to pick up the line in response to a ringing signal, so as to then receive the additional dialling number emitted by the caller, then to generate an action, for example a ring, at the desired telephone terminal.
When installing such a system at a subscriber's home, it is necessary to modify the connection of the telephone terminals on the line so that these terminals do not ring during a telephone call. Indeed, it is the telephone interface circuit that receives the call and then sends a specific ring tone only to the selected terminal. Therefore, the ring of the terminals is inhibited and replaced by a specific ring controlled by the interface circuit, which requires the installation of additional electric connections connecting the interface circuit to each of the specific rings arranged at the terminals.
To avoid installing these additional electric connections, a first known system sends modulated signals to at least one of the wires of the telephone line that connects the various terminals. Thus, some wires of the telephone line are used for a function other than that determined by the telephone network company. But, it may happen that the telephone company does not authorize such a modification of the telephone line connections, even at the subscriber's home.
Another system is described in U.S. Pat. No. 4,653,089 patent but this system requires the use of a specific circuit incorporating a hold coil for imposing a specific voltage level on the line while waiting for additional diallings, and an electric power source.